Printed circuit boards have been implemented as micro substrates or substrate units for carrying chips in semiconductor packages. A plurality of micro substrates are arranged in an array and disposed on a substrate strip where the semiconductor packages are assembled on the substrate strip. Furthermore, a plurality of substrate strips are arranged on a substrate panel for mass production by the processes of printed circuit boards. Normally, before the singulation of a substrate strip, nickel/gold or other metals are electroplated on the metal pads on the substrate panel to prevent metal oxidation and to enhance metal bonding strengths in the following processes such as wire bonding or solder ball placement.
As shown in FIG. 1, a conventional substrate panel 100 comprises a plurality of substrate strips 110, a plurality of current input lines 121 and current output lines 122 at opposing sides, and a plurality of cascaded lines 130 where the lines 121 and 122 and the cascaded lines 130 are copper traces formed from the same metal layer. The substrate strips 110 are connected to each other as one body and are disposed in an array on the substrate panel 100 where some of the substrate strips 110 adjacent to the current input side 101 of the substrate panel 100 are numbered as 110A. The current input lines 121 connect from the current input side 101 of the substrate panel 100 to the adjacent substrate strips 110A, the current output lines 122 from the current output side 102 of the substrate panel 100 to the adjacent substrate strips 110. Two adjacent substrate strips 110 are connected by the cascaded lines 130. During plating processes, current flows from the current input side 101 to the substrate strip 110A through the current input lines 121, then flows to the adjacent substrate strips 110 through the cascaded lines 130, and finally flows to the current output side 102 through the current output lines 122 where the direction of the current flow is shown in the arrows in FIG. 1. Since the current flows to the substrate strip 110A first and then flows to other substrate strips 110, the substrate strips 110A experience the highest current density, then the current density will gradually decrease toward the current output side 102 as the distance of the substrate strips 110 from the current input side 101 increases causing different current densities at different substrate strips 110 at different locations of the substrate panel 100 leading to poor plating qualities such as different plated thicknesses and different plated roughness. Moreover, poor plating qualities will cause wire-bonding failure or poor solder ball jointing strengths during semiconductor packaging processes. Furthermore, when there are current surges or unstable voltages, the adjacent substrate strips 110A to the current input side 101 will be damaged by the surges through the current input lines 121.